1. Technical Field
The present invention relates to a method and apparatus for repairing nuclear reactor core shrouds. In particular, the method and apparatus of the present invention are concerned with repair of boiling water reactor shrouds in order to allow safe reactor operation even with extensive cracking along heat affected zones of horizontal welds.
2. Discussion of the Prior Art
Boiling water reactor shrouds are disposed concentrically within reactor vessels to divide the flow of coolant water through the reactor vessels and to structurally support and align the fuel assemblies, steam separator assemblies and control rod guide tubes. The shrouds are generally cylindrical and are commonly formed from multiple arcuate steel plates having a thickness on the order of one and one-half inches. Plates at each horizontal level of the shroud are joined by welds along abutting, relatively short, vertical edges to encircle the reactor at that level. The plates at different adjacent levels are joined along their abutting, relatively long, arcuate edges with horizontal welds. After periods of use, cracking of the shroud tends to occur within heat affected zones of the welds as a result of corrosion, radiation and stress. Cracking of the vertically oriented welds is considered acceptable because these welds are relatively short in length, relative to the overall shroud length, and do not adversely affect the function of the shroud (i.e., support and alignment of the nuclear fuel assemblies, and channeling of reactor coolant flow). Specifically, vertical welds at adjacent levels are offset angularly about the shroud periphery so that cracking of such a weld can extend, at most, only the axial or vertical length of that level. However, if cracking occurs along the longer horizontal or circumferential welds, a crack can extend along the entire circumference or periphery of the shroud, permitting relative lateral movement between the plate levels. Such excessive cracking, therefore, could prevent the core from supporting and aligning the fuel assemblies, can improperly direct or impede coolant flow, and can permit coolant flow leakage.
When excessive horizontal weld cracking occurs, the shroud must either be replaced or repaired. Repair is certainly the preferred alternative in view of the fact that replacement involves significant expense, relatively long shut down time, and the potential for radiation exposure to personnel. Repair techniques typically involve bolting brackets onto vertically adjacent plates across a weld crack. This approach requires plural brackets for each crack, depending upon the length of the crack. Moreover, welds must be separately inspected after repair for additional cracking of the repaired welds as well as for new cracks in other welds.
A superior method for repairing in situ nuclear reactor shrouds of the type having multiple levels of arcuate plates welded together is disclosed in related copending patent application Ser. No. 08/190,796. The method involves inspecting the shroud for horizontal cracking along welds and securing plural tie-rods in vertical orientation spaced about the periphery of the cracked shroud to axially compress the shroud and thereby urge the opposing surfaces of the horizontal crack toward one another. The repair method is relatively inexpensive, permits safe reactor operation even with excessive cracking of horizontal welds and does not significantly impede coolant flow in the annular space between the shroud and the vessel.